This invention relates to a method of manufacturing a semiconductor device having such a structure that an oxide layer isolates a semiconductor element or part of an element from other elements or parts in a semiconductor substrate, and more particularly to a method of forming at least two PN junctions terminated at such an oxide layer.
In order to provide a compact assembly for a semiconductor device, it is often necessary to isolate a plurality of semiconductor elements and/or two or more active semiconductor regions of a semiconductor element by a relatively thick oxide layer. In such a semiconductor device assembly, it is readily feasible to form a single PN junction to be terminated at the oxide layer. It has, however, been difficult to form two or more PN junctions having their respective ends at the same side of the thick oxide layer. U.S. Pat. No. 3,996,077 discloses a prior art method of manufacturing the above-described semiconductor device. According to this prior art method, a plurality of junctions terminating at a thick oxide layer are formed by introducing impurities from an exposed semiconductor surface. The result is that the respective ends of the junctions, near the thick oxide layer, turn abruptly downward and desired termination thereof cannot be gained. This method will be described later in detail in conjunction with FIG. 1.
A Japanese Patent application, disclosed on Nov. 21, 1970 under Disclosure No. 50-145092 describes another prior art method in which a first PN junction defining a base-collector interface is formed by an ion implantation technique without exposing the surface of a collector region. With this method the deformation of the first junction near the thick oxide layer does not exist. However, a second PN junction, defining a base-emitter interface, is formed by diffusing an impurity from the exposed surface of the base region. Accordingly one end of the second junction, near the thick oxide layer turns abruptly downward and extends over the first junction. Therefore, desired termination of the junction cannot be achieved.
As is apparent from the prior art described above, it has been very difficult to control the distance between the PN junctions near their ends at the thick oxide layer. The difficulty of controlling this distance has resulted in a failure to achieve desired performance and even in contact between the PN junctions, thereby making it impossible to manufacture a semiconductor device.
It is therefore an object of the present invention to provide a reliable method of manufacturing a semiconductor device in which one end of two or more PN junctions is terminated at the same side of a relatively thick oxide layer selectively provided at a principal surface of the semiconductor body.